Oscilation circuit capable of having stable oscilation in wide temperature range

ABSTRACT

An oscillation circuit includes a constant current source, a current mirror circuit configured to receive a constant input current from the constant current source and to output a current proportional to the constant input current, a first inverter configured to be driven with a quartz resonator to oscillate, an operational amplifier configured to supply a power to the first inverter with a voltage equal to an input voltage thereof and a second inverter having a power supply terminal connected to the current mirror circuit and to the operational amplifier and configure to generate the input voltage for the operational amplifier.

This patent application claims priority to Japanese patent application,No. 2004-350626 filed on Dec. 3, 2004 in the Japan Patent Office, theentire contents of which are incorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates to an oscillation circuit capable ofhaving a stable oscillation in a wide temperature range.

BACKGROUND OF THE INVENTION

Electronic circuit systems, particularly a portable system such as, forexample, a wristwatch, a portable phone, and the like, widely use acrystal oscillator to generate a basic clock signal. Such a portablesystem is generally used under a considerably extended range ofenvironmental conditions such as temperature in particular. Therefore,the crystal oscillator used in the portable systems is requested tooperate stably in a wide temperature range.

FIG. 1 illustrates a conventional crystal-oscillator circuit. Theconventional crystal-oscillator circuit includes an inverter INV1, aresistor R1, a quartz resonator XL, two capacitors C1 and C2, and aconstant voltage power supplier 50. The inverter INV1 includes MOSFET(metal oxide semiconductor field effect transistor) transistors andreceives a constant power supply voltage from the constant voltage powersupplier 50. The MOSFET transistors of the inverter INV1 have a transferconductance gm which needs to be constant for a stable oscillation inthis crystal-oscillator circuit. However, characteristics of MOSFETtransistors change in response to variations of an environmentaltemperature, resulting in a change of the transfer conductance gm.

When the transfer conductance gm decreases and, as a result, a loop gainof the crystal-oscillator circuit becomes below “1”, thecrystal-oscillator circuit stops to oscillate because the circuitcondition is out of a range for the oscillation. Meanwhile, if thetransfer conductance gm increases, the circuit may perform an abnormaloscillation. Thus, there is an increasing demand to obtain anoscillation circuit which maintains a stable oscillation in a widetemperature range.

BRIEF SUMMARY OF THE INVENTION

This patent specification describes a novel oscillation circuit whichincludes a constant current source, a current mirror circuit configuredto receive a constant input current from the constant current source andto output a current proportional to the constant input current, a firstinverter configured to be driven with a quartz resonator to oscillate,an operational amplifier configured to supply a power to the firstinverter with a voltage equal to an input voltage thereof and a secondinverter having a power supply terminal connected to the current mirrorcircuit and to the operational amplifier and configure to generate theinput voltage for the operational amplifier.

This patent specification further describes a novel feature of anoscillation circuit where the voltage of the power supplied from theoperational amplifier to the first inverter changes in accordance withtemperature.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the disclosure and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 is a circuit diagram of a conventional crystal-oscillatorcircuit;

FIG. 2 is a circuit diagram of an oscillation circuit according to anexemplary embodiment; and

FIG. 3 illustrates an inverter circuit used in the oscillation circuitshown in FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

In describing preferred embodiments illustrated in the drawings,specific terminology is employed for the sake of clarity. However, thedisclosure of this patent specification is not intended to be limited tothe specific terminology so selected and it is to be understood thateach specific element includes all technical equivalents that operate ina similar manner. Referring now to the drawings, wherein like referencenumerals designate identical or corresponding parts throughout theseveral views, particularly to FIG. 2, an oscillation circuit 1according to an example embodiment is described.

FIG. 2 illustrates a circuit diagram of the oscillation circuit 1according to an exemplary embodiment. FIG. 3 illustrates an invertercircuit used in the oscillation circuit 1 shown in FIG. 2. Theoscillation circuit 1 includes a crystal-oscillator 3 and a power supplycircuit 5. The crystal-oscillator 3 includes a first inverter INV1 and afeedback circuit 7. The feedback circuit 7 includes a resistor R1, aquartz resonator XL and capacitors C1 and C2.

The power supply circuit 5 supplies power to the crystal-oscillator 3from an output terminal. The resistor R1 is connected between input andoutput terminals of the first inverter INV1 so that the first inverterINV1 works as analog amplifier. The quartz resonator XL is alsoconnected between the input and output terminals of the first inverterINV1 so that the first inverter INV1 oscillates at an eigenfrequency.The capacitor C1 is connected between the input terminal of the firstinverter INV1 and a negative power source Vss. The capacitor C2 isconnected between the output terminal of the first inverter INV1 and thenegative power source Vss.

The power supply circuit 5 includes a current source I1, four MOSFET(metal oxide semiconductor field-effect transistor) transistors M1-M4, asecond inverter INV2, and an operational amplifier AMP 2 (differentialamplifier). The second inverter INV2 has a similar circuit configurationto the first inverter INV1 shown in FIG. 3 and includes a P-channelMOSFET M21 and an N-channel MOSFET transistors M22.

The MOSFET transistors M1 and M2 are N-channel MOSFET transistors andthe MOSFET transistors M3 and M4 are P-channel MOSFET transistors. TheMOSFET transistors M1 and M2 form a current mirror circuit byconnections of their gates to each other and their sources to thenegative power source Vss. The gate of the MOSFET transistor M1 isconnected to a drain of the MOSFET transistor M1. The drain of theMOSFET transistor M1 is connected to a positive power source Vdd throughthe current source I1 so that a drain current of the MOSFET transistorM2 is proportional to the current of the current source I1.

The MOSFET transistors M3 and M4 also form a current mirror circuit byconnections of their gates to each other and their sources to thepositive power source Vdd. The drain of the MOSFET transistor M3 isconnected to the drain of the MOSFET transistor M2 so that the draincurrent of the MOSFET transistor M3 is equal to the drain current of theMOSFET transistor M2. Additionally, the gate of the MOSFET transistor M3is connected to the drain of the MOSFET transistor M3 so that thecurrent mirror circuit outputs a drain current I4 of the MOSFETtransistor M4 proportional to the current of the current source I1.

The MOSFET transistor M4 has a drain connected to a power supplyterminal of the second inverter INV2 which has input and outputterminals shorted. A voltage applied through the drain of the MOSFETtransistor M4 to the power supply terminal of the second inverter INV2is referred to as a power supply voltage Vin. With this configuration, aconstant current which is a drain current of MOSFET transistor M4 flowsto the power supply terminal of the second inverter INV2. As a result,the MOSFET transistor M21 and the MOSFET transistor M22 of the secondinverter INV2 are driven into a saturation region of the MOScharacteristics.

A transfer conductance gm of the MOSFET transistor in the saturationregion is generally defined as a function of drain current Id andsatisfies a formula 1:gm=√(2k*1d),in which k is determined by a manufacturing process and a transistorsize.

When the current generated by the current source I1 is constant overvarying temperature, the transfer conductance gm has an approximatelyconstant value over varying temperature.

The transfer conductance gm of MOSFET transistor in the saturationregion can also be defined by a potential difference between agate-source voltage Vg and a threshold voltage Vt of the MOSFETtransistor. More specifically, each of the MOSFET transistors M21 andM22 has the transfer conductance gm defined by a potential differencebetween the gate-source voltage vg and the threshold voltage Vt thereof.The transfer conductance gm generally satisfies a formula 2:gm=k(Vg−Vt),in which Vg is the gate-source voltage of the MOSFET transistors M21 andM22 and Vt is the threshold voltage of the MOSFET transistors M21 andM22.

The power supply voltage Vin of the second inverter INV2 is a sum of thegate-source voltages of the P-channel MOSFET transistor and N-channelMOSFET transistor. The Vt has a temperature dependence. Based on theformula 2, the potential difference Vg of the gate-source of the MOSFETtransistor has a temperature dependence because the transfer conductancegm is constant in this exemplary embodiment. The power supply voltageVin of the second inverter INV2 changes in accordance with a change of atemperature.

In this embodiment, the power supply voltage Vin of the second inverterINV2 is fed to a non-inverting input terminal of the operationalamplifier AMP2. The operational amplifier AMP2 is configured to operateas a voltage follower circuit since its output terminal is connected toan inverting input terminal thereof. The output voltage Vout is thepower supply voltage for the inverter INV1 of the crystal-oscillator 3.Therefore, the same voltage as the power supply voltage Vin of thesecond inverter INV2 is fed to a power supply terminal of the inverterINV1 of the crystal-oscillator 3.

Moreover, the MOSFET transistors M21 and M22 which form the secondinverter circuit INV2 is designed to have a constant transferconductance gm against variations of temperature. Additionally, thefirst and second inverter circuits INV1 and INV2 have a commonconfiguration so as to have approximately equivalent electrical andtemperature characteristics.

The power supply voltage Vout of the first inverter INV1 is equal to thepower supply voltage Vin of the second inverter INV2 so that thetransfer conductance gm of the first inverter circuit INV1 is equal tothe transfer conductance of the second inverter circuit INV2. As aresult, the transfer conductance gm of the first inverter is keptconstant with regard to temperature. Consequently, the oscillationcircuit 1 maintains a stable oscillation with a continuous and noabnormal oscillation in a wide temperature range.

Numerous additional modifications and variations are possible in lightof the above teachings. It is therefore to be understood that within thescope of the appended claims, the disclosure of this patentspecification may be practiced otherwise than as specifically describedherein.

1. An oscillation circuit comprising; a constant current source; acurrent mirror circuit configured to receive a constant input currentfrom the constant current source and to output a current proportional tothe constant input current; a first inverter configured to be drivenwith a quartz resonator to oscillate; an operational amplifierconfigured to supply a power to the first inverter with a voltage equalto an input voltage thereof; and a second inverter having a power supplyterminal connected to the current mirror circuit and to the operationalamplifier and configure to generate the input voltage for theoperational amplifier.
 2. The oscillation circuit according to claim 1,wherein the second inverter has input and output terminal connected toeach other.
 3. The oscillation circuit according to claim 1, wherein thevoltage of the power supplied from the operational amplifier to thefirst inverter changes in accordance with temperature.
 4. Theoscillation circuit according to claim 1, wherein the second inverterincludes an N-channel MOSFET transistor and a P-channel MOSFETtransistor and has electric characteristics and temperature dependencesubstantially equivalent to characteristics and temperature dependenceof the first inverter.